1. Field of the Invention
The present invention relates to methods for heating the glass contacting surfaces of glass forming equipment to a proper working temperature, and to an improved hydrocarbon fuel gas mixture especially suited for said methods and lubricating purposes.
2. Description of the Prior Art
Pressed glass articles, e.g., tableware, lenses, glass blocks, white ware and crystal ware, are usually made by placing a charge of heat-softened glass, commonly known as a “gob”, in the cavity of a mold, and then advancing a plunger, under pressure, into the mold. Modern manufacturing processes, which generally enable large quantities of pressed glass articles of precise dimensions to be produced in a relatively short time, usually employ a plurality of molds. The cavities of the molds are charged with gobs of softened glass from a central reservoir, and the gobs are formed into pressed articles by advancing into the molds plungers driven, for example, hydraulically or by air. Typically, automated pressing operations are used. In such operations several molds are mounted on a circular table and rotated step by step to bring each mold, in turn, below a charging device, then to a pressing station and, finally, to a station where the pressed articles are removed from the molds.
Glass is also formed in press and blow machines, such as the H-28 press and blow machine, which used to have twelve stations, but now have eighteen stations. Achieving and maintaining the glass contacting surfaces of press and blow machines is critical to producing quality glassware.
Heretofore, molds, plungers and other glass contacting elements of glass forming equipment have been heated, prior to forming glassware, by depositing gobs of glass on the surfaces and allowing the glass to heat the glass contacting element to a good working temperature. This heating has been augmented, in some cases, with steam or the combustion of natural gas or ethyl acetylene. In glass forming equipment with cast iron glass contacting elements, 600° F. is a good working temperature. More recently, molds, plungers and other glass contacting elements of glass forming equipment have been made from stainless steel which has a much higher working temperature of above 800° F. The prior art reliance on gobs of glass, with or without supplemental heating, made the time required to heat glass contacting elements to a suitable working temperature dependent upon the mass of the glass. Typically, this takes about thirty minutes.
Today, commercial glassware production involves many short, but fast, runs of glassware with intermediate changes in molds and the like. Consequently, today, a higher proportion of glass forming time is required to bring glass contacting elements up to working temperatures. As a result, it is more important than ever to expedite the heating of glass contacting elements of glass forming equipment to a suitable working temperature.
Particularly in automated glass pressing operations, a major consideration has been the need to heat glass contacting elements to a working temperature to prevent formed glass from crizzling due to insufficient working temperature. If the temperature of the mold or plunger or other glass containing surface is not maintained as a suitable working temperature, for example, 800° F. for stainless steel elements, or is permitted to fall below a suitable working temperature during a run, wrinkles, checks and other imperfections can result in the finished article.
Today, some glass contacting elements are heated by contact with glass gobs and the combustion of a mixture of combustible gases, namely, 75% by volume of MAPP gas, a mixture of methylacetylene, propadiene and propylene, which is commercially available from Petromont of Canada, and 25% by volume of propane. Better heating of glass contact surfaces is needed, however, especially in view of the tendency toward shorter runs and longer downtime while glass contacting elements are brought up to a suitable working temperature according to the prior art.
In glass operations today, the main hydrocarbon used is acetylene gas. Because of the complex chemical attributes of acetylene, hot spots in molds and other glass contacting surfaces and carbon breakdown cause major problems in production runs and air quality standards. Because acetylene is a 100% triple bonded carbon, the release of heat energy and carbon combustion is too abundant for many types of mold compositions.
Furthermore, in today's glass plant operations, the variety of glass items, the variety of mold materials, and the increased short runs makes the requirements for quick mobility and instant operation production very essential. The set point must be attained quickly with a gas flame system.
It is a desideratum of the present invention to avoid the animadversions of the prior art.